1. Field of the Invention
The present invention relates to a wireless communication apparatus, a wireless communication method, and a computer program for performing an antenna calibration process compensating an imbalance in phase and amplitude present between antenna branches of a plurality of antennas. More specifically, the present invention relates to a wireless communication apparatus, a wireless communication method, and a computer program for performing the calibration process on the antenna branches in a communication system handling a wide-band signal.
The present invention also relates to a wireless communication apparatus, a wireless communication method, and a computer program for performing the calibration process on the antenna branches in a wide-band communication system, such as an orthogonal frequency division multiplexing (OFDM) system, which divides a wide-band signal into a plurality of sub-carriers. In particular, the present invention relates to a wireless communication apparatus, a wireless communication method, and a computer program for performing the calibration process on the antenna branches in a wide-band communication system that uses a plurality of packet formats different in the sub-carrier placement on a frequency axis as defined in the IEEE802.11n standard.
2. Description of the Related Art
A wireless network draws attention as a system that is free from wiring in wired communication method in the related art. Standards available for wireless network are the Institute of Electrical and Electronic Engineers (IEEE) 802.11 and IEEE802.15. For example, IEEE802.11a/g as wireless LAN standard adopts multi-carrier OFDM modulation method. The IEEE802.11a/g standard supports a communication system of up to a communication speed of 54 Mbps. There is a need for a next-generation wireless LAN standard that can achieve an even higher bit rate.
A multi-antenna technique may be available as a wireless communication technique to achieve a high-throughput wireless data transmission. In the multi-antenna technique, a communication apparatus works on a plurality of antennas. An adaptive array antenna is widely known as a multi-antenna technique. The adaptive array antenna supports communications by controlling a gain of each antenna element and determining antenna directivity appropriate for transmission and reception. More specifically, a signal received by each antenna element is multiplied by an appropriate weight to obtain a receiving directivity pattern of the entire array antenna. The transmission signal is then multiplied by an appropriate weight of each antenna element and then transmitted from the respective antenna element. A transmission directivity pattern as the entire array antenna is thus controlled. The array antenna may work as a sector antenna in which a main lobe of the antenna is directed to a desired direction with a low-level side lobe directed to an undesired direction so that no radio wave is transmitted in the undesired direction. In another method, the main lobe of the antenna is directed to a desired mobile station and a null lobe of the antenna is directed to an interfering mobile station so that a signal-to-interference and noise power ratio (SINR) is increased.
Multi-input and multi-output (MIMO) communication also draws attention as another example of wireless communication techniques employing the multi-antenna. The MIMO communication provides a spatial multiplexed stream by arranging a plurality of antenna elements in each of a receiver side and a transmitter side. The transmitter side multiplexes transmission data by spatial coding and time coding a plurality units of transmission data, and sending the transmission data units to a plurality of transmitting antenna elements to transmit the transmission data units via channels. The receive side then spatial decoding and time decoding signals received via a plurality of receiving antenna elements via the channels, demultiplexes the decoded signals into a plurality of units of data, and thus obtains the original data in a manner free from crosstalk between streams. Without widening the frequency band, the MIMO communication method increases a capacity of transmission in response to the number of antenna elements and increases the communication speed. Since spatial multiplexing is used, frequency utilization is high. The MIMO communication method takes advantage of channel characteristics and is different from a mere transmission and reception adaptive array. For example, the IEEE802.11n standard as an expanded version of the IEEE802.11 standard adopts the OFDM_MIMO communication method.
When a radio frequency (RF) signal passes through an RF transmitter circuit or an RF receiver circuit in the multi-antenna system, an imbalance in phase and amplitude between antenna branches appears as the effect of individual variations in an active element and a component, such as an amplifier, a frequency converter, or the like, forming the RF transmitter circuit or the RF receiver circuit, regardless of the type of multi-antenna technique. The effect of individual variations is particularly great in an automatic gain control (AGC) circuit in the RF receiver circuit and a power amplifier (PA) in the RF transmitter circuit. The imbalance in phase and amplitude between branches leads to degradation in antenna characteristics of the adaptive array, and may result in a directivity completely different from an intended directivity. The imbalance in phase and amplitude between branches in the MIMO communication causes an erratic channel recognition, and a failure in the acquisition of appropriate transmission channel forming row and column. Decoding characteristics on the receiver side are thus substantially degraded.
A calibration process is performed to equalize characteristics between the RF transmitter circuit and the RF receiver circuit in order to control the effect of imbalance in phase and amplitude. The calibration process is typically performed in the frequency domain. In each branch, a calibration coefficient is multiplied on a per sub-carrier basis in the frequency domain.
A communication system handling a wide-band signal acquires the calibration coefficient for each frequency band in use. For example, the OFDM system divides a wide-band signal into a plurality of sub-carriers using fast Fourier transform (FFT) as previously discussed. In this case, the antenna calibration coefficient is determined on a per sub-carrier basis. A transfer function is acquired through transmission and reception of a packet containing all the sub-carriers, and the antenna calibration coefficient is determined for each sub-carrier.
For example, Japanese Unexamined Patent Application Publication No. 2005-348236 discloses an array antenna transmission apparatus. In accordance with the disclosure, the whole frequency band is divided into a plurality of blocks, and amplitude/phase deviations of the sub-carriers are averaged over all the sub-carriers within one block on the basis of the fact that the deviation of a frequency response between adjacent sub-carriers is small. The array antenna transmission apparatus thus obtains a highly accurate calibration coefficient.
Referring to FIG. 7, the IEEE802.11n standard adopting the OFDM_MIMO communication method contains a total of five packet formats. The five packet formats include (1) Legacy Mode working on the 20 MHz band, (2) HT Mixed Mode working on the 20 MHz band, (3) HT Mixed Mode working on the 40 MHz band, (4) 40M Duplicate Legacy Mode working on the 20 MHz band (lower band) of the 40 MHz band and the 20 MHz band (upper band) of the 40 MHz band in a duplicate fashion, and (5) 40M Duplicate HT Mixed Mode working on the 20 MHz band (lower band) of the 40 MHz band and the 20 MHz band (upper band) of the 40 MHz band in a duplicate fashion. Sub-carriers are different on the frequency axis depending on each of a packet format having signals within the 20 MHz band, a packet format having signals within the whole 40 MHz band, a packet format having signals in the upper band of the 40 MHz band, a packet format having signals in the lower band of the 40 MHz band, and a packet format having signals in each of the upper band and the lower band of the 40 MH band. Five calibration cycles are performed to obtain the calibration coefficients for the five packet formats.
Normal communications are not performed during the calibration operation.